Radome structures are conventionally used to protect microwave antennas from the physical environment. It is also desirable to shield such equipment from external incident electromagnetic energy which can adversely affect the electrical operating characteristics thereof. Such a shield, during the operation of the antenna equipment, should be transparent to the energy only in the selected frequency range handled by the antenna equipment. However, when the antenna equipment is not operating, such a shield should reject electromagnetic energy within such frequency range as well as outside such frequency range.
Radome shields having such characteristics are often referred to as "shutter-type" radomes, the shutter being effectively "closed" to all frequencies both within and outside the frequency band of interest during non-operation and the shutter being effectively "open" only to frequencies in the desired operating frequency band during operation.
One proposed shutter arrangement is described in currently copending U.S. patent application Ser. No. 512,260 of Jean-Claude Sureau filed Sep. 7, 1982. Such structure can be effectively described as a "transmission resonant" shutter which operates in a manner such that during a first operating mode (i.e., an "open" shutter mode) energy is permitted to be transmitted through the structure within a selected frequency range, the shutter panel thereof being essentially resonant during such transmission mode. During the second operating mode (the "closed" shutter mode) the shutter panel is non-resonant and transmission of energy both over the selected frequency range and outside the selected frequency range is substantially small.
Another proposed shutter arrangement is described in U.S. patent application Ser. No. 527,029 filed by Jean-Claude Sureau on Aug. 9, 1983. Such structure can be effectively described as a "non-resonant" shutter structure which operates so that during a first operating mode (the "open" shutter mode) transmission is permitted over a relatively wide range of frequencies, normally arranged to extend from the low end of the frequency spectrum to a selected higher frequency. Transmission is substantially prevented above the selected frequency. During such mode, the shutter panel does not operate as a resonant structure. During the second operating mode (the "closed" shutter mode) the structure prevents the transmission of energy substantially over the entire frequency spectrum and again does not operate as a resonant structure.
While the above structures have their uses in certain applications, the structures are relatively expensive since they utilize a relatively large number of diodes for the shutter structure and operation. Moreover, during the closed shutter mode the suppression of energy transmission may not be adequate in applications which require a greater degree of energy suppression, particularly in the specific selected frequency range of interest.
It is desirable, therefore, to provide a structure which has improved suppression characteristics over a selected frequency range in the "closed" shutter mode and to provide such operation at reduced cost over that provided by the previous systems.